Loudspeakers are known in many different variants. A conventional type of loudspeaker is the piston type loudspeaker, having a vibrating cone to drive the air, so that a beam of sound can be generated. In many applications, the conventional loudspeaker comprises a relatively large cabinet extending behind the vibrating cone, to prevent that backwardly radiating sound can enter the same space as an emitted beam of sound.
Another type of loudspeaker is the so called “flat panel” speaker, see for example U.S. Pat. No. 6,481,173 B1. The known flat panel speaker comprises a radiating panel, and an exciter hooked up to the panel to cause the panel to vibrate. Use is made of a complex random ripple of wave forms on the panel surface, leading to an omni-directional sound generation. Compared to piston-like movement of the conventional cone-type loudspeaker, the motion of the flat panel speaker is determined by many different modes spread over the radiator surface, possibly leading to incoherent sound radiation. A common disadvantage of known flat panel speaker systems is that they are not good in producing low frequency sound.
U.S. Pat. No. 4,322,583 discloses a prior art honeycombed core structure of a flat plate electroacoustic transducer.
JP59083497 relates to a diaphragm for a speaker, which is lightweight and highly rigid. JP'497 aims to improve disadvantages of known honeycomb-type core diaphragms, and provides a sheet which has a large number of hollow protrusions, uniform and concentric with the centre of diaphragm.
Particularly, according to the description of JP'497, FIG. 1 of JP'497 is a plan view which is partially cut away of the surface material of the speaker diaphragm presented in an exemplary embodiment of the present invention, and FIG. 2 of JP'497 is a view in cross section. In these figures, (1) is a surface material, and (3) is a metal or plastic sheet which comprises hollow cylindrical protrusions (2); said protrusions (2) are concentric with respect to the centre of the diaphragm and arranged uniformly in the circumferential direction, and they are integral with the sheet (3). Furthermore, the bottom surfaces of the protrusions (2) are through-holes. According to JP'497, the surface material (1) is adhered to the tip end surface of the cylindrical protrusions (2) of the abovementioned sheet (3) by means of a polyamide-based or EOA-based adhesive. The surface material (1) can be a metal or plastic sheet.
Moreover, JP'497 provides a description of a production method, in which the sheet (3) comprising the protrusions (2) is moulded using a moulding method such as vacuum forming, extrusion moulding or press moulding to mould the metal or plastic sheet described above, after which the tip end side of the protrusions (2) is slightly cut to open the protrusions (2). After this, the sheet-like surface material (1) is adhered to the tip end surface of the protrusions (2) by means of adhesive, whereby it is possible to produce the diaphragm.
Moreover, according to JP'497, the end surfaces of the protrusions (2) can be open or closed. In the latter case, the step of cutting the end surfaces can be dispensed with, and this has an effect whereby the step of adhering the protrusions (2) to the surface material (1) becomes simpler.